This invention relates to photoactive semiconductor electrodes. More particularly, it is concerned with a method of preparing n-type semiconductor electrodes utilizable in electrochemical cells and in photoassisted electrochemical oxidation reactions.
There has been considerable recent interest in the application of photoactive semiconductor electrodes to the electrolysis of water and to the direct conversion of solar to electrical or chemical energy. The uses of such electrodes have recently been generalized to reduction-oxidation reactions in addition to the electrolysis of water. Oxidation reactions at n-type photoactive semiconductor electrodes and reduction reactions at p-type electrodes can be carried out at potentials much lower than ordinarily required using light as the driving force for the reactions. Such processes have been termed "photoassisted" rather than photocatalyzed reactions.
It is known that electrodes fabricated from single crystals of pure titanium dioxide, doped single crystals of titanium dioxide, or polycrystalline titanium dioxide deposited on an appropriate substrate can be used as photoelectrodes. Titanium dioxide normally has high electrical resistivity. To form electrically conductive semiconductor material the titanium dioxide is typically treated by reduction with hydrogen. It is theorized that such treatment produces a material with oxygen lattice deficiencies in the titanium dioxide crystal with the lattice defect sites contributing to the semiconductor properties. This partially reduced material can be characterized by the general formula TiO.sub.( 2&lt;x) where x takes on a value between zero and one. These partially reduced phases of titanium dioxide are called Magneli phases of titanium dioxide.
Because of the great possibilities which these electrodes have for the conversion of light to electrical or chemical energy, a number of studies have been directed to methods of fabricating electrodes which make such conversions more efficient. In previously described uses of n-type titanium dioxide semiconductor electrodes, it has generally been the practice to use electrodes formed of single crystals of TiO.sub.2 or of polycrystalline TiO.sub.2, reduced to the Magneli phases.
The technique of producing single crystal photoactive TiO.sub.2 electrodes is described, for example, by S. N. Frank et al. in "Semiconductor Electrodes 11. Electrochemistry at n-Type TiO.sub.2 Electrodes in Acentonitrile Solutions", J. Am. Chem. Soc., 97:7427 (1975). Polycrystalline titanium dioxide electrodes produced by chemical vapor deposition techniques are treated by K. L. Hardee et al. in "The Chemical Vapor Deposition and Application of Polycrystalline n-Type Titanium Dioxide Electrodes to the Photosensitized Electrolysis of Water", J. Electrochem. Soc., 122:739 (1975).
Single crystal TiO.sub.2 electrodes or doped single crystal TiO.sub.2 electrodes are often costly and difficult to produce. On the other hand, polycrystalline electrodes which utilize Magneli phase TiO.sub.2 as the photoactive semiconductor material are less difficult and costly to produce, but are limited in their spectral response to wavelengths of light shorter than about 450 nanometers.
It is therefore an object of the present invention to provide a simple and improved method of fabricating modified polycrystalline photoactive semiconductor electrodes utilizable in photoelectrochemical cells and electrochemical oxidation reduction reactions which are photoactive at wavelengths longer than 450 nanometers. The method comprises the steps sequentially of applying a suspension of finely divided electrode coating material to the surface of a metal body wherein the electrode coating material comprises titanium dioxide and at least one oxide of a metal selected from the group consisting of aluminum and d-electron transition metals excluding titanium, and heating the coated metal body at an elevated temperature for a period of time sufficient to sinter the electrode coating to the metal body.